In a conventional display apparatus such as an HMD (Head Mounted Display), a system (to be referred to hereafter as a beam scanning system) of scanning a laser beam two-dimensionally and drawing the scanned laser beam directly onto a retina of an eye may be employed (see Patent Document 1, for example). A display apparatus used in the beam scanning system is known as a retinal scanning display, a retinal irradiation display, a retina direct drawing display, a beam scanning display, a direct vision display apparatus, an RSD (Retinal Scanning Display), a VRD (Virtual Retinal Display), and so on.
FIGS. 1A and 1B show a constitutional example of an eyeglass-type HMD. As shown in FIGS. 1A and 1B, the eyeglass-type HMD includes laser light sources 101/110 carried on an eyeglass frame to emit laser beams, wavefront shape modification units 102/109 for controlling wavefronts of the laser beams, and scanning units 103/108 for scanning the laser beams in a two-dimensional direction.
The laser beams emitted from the laser light sources 101/110 are projected toward eyeglass lenses by the scanning units 103/108. The laser beams are then reflected by deflecting units 104/107 provided on surfaces of the eyeglass lenses so as to enter the eyes of a user and form an image on retinas. A half-mirror, an HOE (Holographic Optical Element), or similar is used as the deflecting units 104/107. Thus, the deflecting units 104/107 allow the user to see both an outside view and an image depicted by the laser beams at the same time. A mirror device or the like that scans a laser beam in a two-dimensional direction by causing a single-plate mirror to oscillate in a uniaxial or biaxial direction is used in the scanning units 103/108.
Further, a method of providing the image displayed to the user with a sense of perspective by modifying a wavefront curvature of the laser beam may be considered (see Patent Document 2, for example).
However, to realize a wide viewing angle and a high resolution in a beam scanning display apparatus, a beam waist position of the scanned laser must be modified in accordance with the laser beam scanning operation. Note that in this specification, the beam waist position is expressed as a distance between a beam waist of the scanned laser and a scanning unit.
To realize a high resolution in a beam scanning display apparatus, a beam spot diameter on the retina must be reduced. A human eye functions as a condenser lens. Therefore, when a parallel laser beam (a laser beam with an infinite radius of curvature) having a large beam diameter enters the eye, the incident light is condensed into a small spot on the retina of the human eye. As a result, a high-resolution image can be drawn on the retina. To form a beam entering the eye of the user into a parallel beam in an eyeglass-type beam scanning display apparatus, the beam waist position of the scanned laser must be modified in accordance with a movement of the scanning unit. In other words, the beam waist position of the scanned laser is modified in accordance with a location in which the scanned laser scanned by the scanning unit enters the deflecting unit. Thus, a laser beam traveling toward the eye of the user from the deflecting unit can be made into a parallel beam.
FIG. 2 shows an example of an optimum beam waist position for making a laser beam traveling toward the eye of the user into a parallel beam in an eyeglass-type display apparatus.
As shown in FIG. 2, the scanning unit 103 causes a laser beam L to enter the deflecting unit 104 diagonally. A hologram mirror designed to exhibit an action for condensing light into a pupil position 133 of the user is used as the deflecting unit 104.
Here, an appropriate beam waist position of the laser beam L varies according to the position in which the laser beam L enters the deflecting unit 104. In the drawing, a trajectory indicated by a dotted line 201 shows an example of an appropriate beam waist position of the laser beam L. More specifically, by performing control such that the beam waist position of the laser beam L remains positioned on the trajectory shown by the dotted line 201 even if the orientation of the laser beam L is altered by a movement of the scanning unit 103, the beam that enters the human eye can be made into a parallel beam.
To display a high-quality image, the scanning unit 103 must be moved at high speed (100 Hz or more, for example). In this case, the beam waist position of the scanned laser must be modified at high speed in accordance with the operation of the scanning unit 103. Therefore, a method of modifying the beam waist position of the laser beam at high speed by subjecting an optical component to simple harmonic driving, for example, is employed.
FIG. 3 shows a constitutional example of the principal parts of a conventional beam scanning HMD.
As shown in FIG. 3, the wavefront shape modification unit 102 includes lenses. These lenses 102a/102b are respectively subjected to simple harmonic driving in synchronization with the movement of the scanning unit 103. As a result, a horizontal direction beam waist position and a vertical direction beam waist position of the scanned laser are modified.
However, in a method where the beam waist position of the laser beam is modified at high speed by subjecting a lens or the like to simple harmonic driving, such as the conventional method described above, an actuator or the like for driving the lens is required, leading to an increase in the size of an optical system of the HMD. In an eyeglass-type HMD, the optical system needs to be small from the viewpoints of wearability and design, and it is therefore difficult to respond to this requirement with a lens driving method.
Further, with a method in which the lens position is modified using an actuator, an actual beam waist position may deviate greatly from an ideal beam waist position when an operation of the scanning unit deviates from an operation of the actuator. When a servo function is introduced to adjust this deviation, the size of the optical system increases even further.
In Patent Document 1 and Patent Document 2, however, the aforesaid problems regarding the size of the optical system for controlling the beam waist position are not taken into consideration.